A capacitor microphone is a type of microphones which have a sort of a capacitor formed by a assembly of a vibrating plate and a fixed electrode to extract electric signals, in which the electric signals are generated from capacitance change in response to the displacement of the vibrating plate vibrated by a sound wave. In capacitor microphones of the prior arts, it is required that DC voltage is supplied across the vibrating plate and the fixed electrode. The DC voltage is called as polarization voltage. There are two methods for supplying the polarization voltage, one is supplying DC voltage out of the microphone, the other is supplying polarization voltage by electret in the microphone.
A conventional capacitor microphone uses an impedance converter such as an FET (Field Effect Transistor) or a vacuum tube to obtain a specific level of a voltage signal, because impedance between the vibrating plate and the fixed electrode is much higher.
However, a conventional capacitor microphone described above is easily affected by outside electric or magnetic fields when capacitance change is converted to voltage, so that noise may be generated. Further, a conventional capacitor microphone uses an impedance converter in which charge leakage inherent thereto may be occurred, for example, when humidity is high. The charge leakage may also generate noise.
A conventional capacitor microphone using electret in a microphone unit to supply polarization voltage is enable to be produced small-sizedly, for example, to use for a cellular phone, however, when the capacitor microphone is mounted on a substrate, the reflow-soldering method may not be used because electret is affected by heat. Therefore, electret is required to be mounted separately on the substrate after chip resistors and capacitors have been mounted, so that the productivity of these capacitor microphones is low.